Hermetically sealed variable resistors and potentiometers



Nov. 25, 1958 w. J. MAIRS 2,862,088

HEIRMETICALLY SEALED VARIABLE RESISTORS AND POTENTIOMETERS Filed Feb. 11, 1958 F l 6 INVENTOR.

f w/m/W INA/k8 ATTORNEYS United States Patent HERMETICALLY SEALED VARIABLE RESISTORS AND PQTENTIOMETERS William J. Mairs, Waltham, Mass, assignor to Acton Laboratories, Inc., Acton, Mass., a corporation of Massachusetts Application February 11, 1958, Serial No. 714,575

Claims. (Cl. 201-55) The present invention relates to variable resistors or potentiometers and this application is a continuation in part of my copending application Ser. No. 567,613, filed February 24, 1956, for Variable Resistor or Potentiometer. The present invention is an improvement over the invention described and claimed in my copending application.

My copending application describes and claims a variable resistor or potentiometer comprising a base, a resistor on said base, a resilient conductive member in spaced overlying relation with the resistor, a resilient insulating member in overlying engaging relation with the conductive member, said insulating member being hermetically sealed to said base to protect the resistor and conductive member from the atmosphere, and a slide or pressure foot acting on the insulating member to press the conductive member into contact with the resistor.

The primary object of the present invention is to provide a simple means for lubricating the exposed surface of the resilient insulating member so as to facilitate sliding of the pressure foot over the exposed surface. It is to be noted that a predetermined pressure is required to be exerted by the foot on the resilient insulating memher for the conductive layer to properly engage the resistor. This pressure creates friction between the insulating member and the pressure foot which tends to oppose movement of the pressure foot. Backing the pressure foot off to reduce this friction is not feasible since then the pressure foot would not exert the required predetermined pressure on the insulating member. Reducing the size of the portion of the pressure foot that engages the insulating layer will reduce the friction but this can be done only to a limited extent without reducing the strength and effectiveness of the pressure foot. However, it has been recognized that the friction may be reduced without reducing the pressure exerted by the pressure foot by reducing the coeificient of friction between the surfaces of the insulating member and the pressure foot. This can be effected by 1) choosing a material having a smooth slippery surface, or (2) by coating a given material with a suitable low friction substance such as a plastic. Both of the approaches are considered in the aforementioned copending application, the application disclosing (l) the use of a polyester film which has a relatively smooth slippery surface and (2) the use of a glass fibre fabric provided with a smooth coating of tetrafiuorethylene resin. The tetrafiuorethylene resin not only presents a smooth surface but itself acts as a lubricant.

The present invention reduces the coefiicient of friction in a third way, namely, by providing a supply of lubricant which is applied in small amounts to the surface of the flexible resilient insulating member.

Accordingly, a more specific object of the present invention is to provide a supply of lubricant which is applied to the surface of the resilient insulating member in amounts and at a frequency dependent upon the length "ice of travel and the frequency of operation of the pressure foot.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Fig. l is a vertical section taken through the center of a potentiometer embodying the present invention;

Fig. 2 is an enlarged view of a portion of Fig. 1;

Fig. 3 is an exploded view of the resistor unit;

Figs. 4 and 5 are detailed views of the pressure arm employed in the device of Fig. l; and

Fig. 6 is an enlarged view of a portion of Fig. 2.

Referring now to Fig. 1, there is shown a rotary potentiometer which, except for the improvement provided according to the present invention, is substantially the same as the rotary potentiometer shown in my copending application Ser. No. 567,613. In this figure there is shown a cylindrical housing 2 having a fixed wall 4 at one end and a removable end cover 6 at the other end. The housing is provided with a suitable bearing structure 8 for supporting a rotatable actuating shaft 10. The inner end of actuating shaft 8 is provided with a radially extending arm 12. As seen in Figs. 4 and 5, arm 12 is bifurcated to provide two parallel extensions 14 which are adapted to be drawn together by a cap screw 16 so as to clamp the arm to shaft 10. Mounted on the free end of arm 12 is a contact pressure foot 18. Pressure foot 18 has a rounded tip 20 which bears against the upper surface of a resistance unit 22 which is posi tioned on a support ring or washer 24. Both resistance unit 22 and washer 24 are held in concentric relation with shaft 10 by means of a retaining ring 26. Bearing member 8 has a circumferential groove 28 in which a suitable adhesive or cement 30 is applied to hold retaining ring 26 in place. A second circumferential groove 32 is formed in retaining ring 26 for the purpose of receiving additional cement 30 for securing it to resistance unit 22 and washer 24. The latter in turn has annular grooves 34 and 36 formed in its two faces for receiving cement 30 to secure it to resistance unit 22 and the adjacent end wall 4 of the housing 2.

The resistance unit 22 is shown in exploded form in Fig. 3. It comprises an annular base member 40 of suitable insulating material, such as glass or quartz, upon one face of which there has been deposited an annular strip of metal film 42 to serve as a resistance element. The metal film extends through an are less than a full circle and at its opposite ends suitable terminations or contact portions 44 and 46 are provided for making electrical contact to an external circuit. A third contact element 48 is provided on the peripheral surface of base 40 for a purpose hereinafter explained.

Positioned on base 40 are two annular washers or gaskets 50 and 52 of suitable insulating material, such as kraft paper. ,Washer 50 surrounds the resistance film 42 and the latter surrounds washer 52. The thickness of gaskets 50 and 52 can range from .0015 to .035 inch, with .003 inch being found best in many cases. Overlying washers 50 and 52 and supported thereby in spaced relation to base 40 and resistance film 42 is an annular resilient metal foil contact element 54 having a projecting tab 56 which engages the contact portion 48. Tab 56 is secured to contact portion 48 by soldering or by a suitable conductive cement. The conductive member 54 may comprise a sheet of coin silver or other highly conductive material including precious metal alloys. Beryllium copper plated with rhodium has been employed. Immediately above conductive member 54 is an annular v flexible resilient insulating member 60, preferably made of fibreglass cloth impregnated with Teflon. Alternatively, insulating member 69 may be made of other suitable materials such as a polyester film. Preferably the adjacent faces of the conductive member 54 and the insulating member 6%) are secured together by means of a suitable adhesive 61, so that the two members will-flex together with the insulating member functioning to strengthen the foil against cracking under repeated flexing.

Suitable adhesive -is applied to both sides of the ahnular gaskets S and 52 to hold the elements 46, 56, -2, 54 and 60 together as a single resistance unit wherein the resistance film 42 and the conductive member 54 are hermetically sealed ofl from the atmosphere.

Of course, it will be appreciated that although the conductive member 54 is shown as a separate element constructed of metallic foil, it could take the form of a conductive coating on the'fibreglass insulating member 68.

Preferably the outside diameters of the retaining ring and the hermetically sealed resistance unit 22 are of such a magnitude as to provide a narrow space between those elements and the interior surface of the housing which is filled with a suitable potting compound 62. The potting compound 62 not onlysupports the resistance unit 22 and the ring 24 against damage due to shock, but also acts to further hermetically seal together the edges of the various elements shown in Fig. 3 which make up the resistance unit 22.

The adhesive 30 and the potting compound 62 may be the same material or different materials. Preferably these comprise a thermosetting resin, e. g. an epoxy resin, which are baked so as to be thoroughly cured. Also suitable for the same purpose is a self-vulcanizing silicone rubber compound, such as Dow Corning RTV 501.

The three contact portions 44, 46 and 43 are connected to three leads 66 which extend through a suitable opening in housing 2 and are connected to separate terminal posts 68 mounted on the exterior of the housing.

To the extent already described the potentiometer illustrated in the drawing is substantially the same as the rotary potentiometer illustrated in my copending application Ser. No. 567,613. The pressure foot 18 acts on the insulating member 60 with just enough pressure to cause a limited portion of conductive member 54 to engage a small area or segment of the resistance element 42. As the shaft is rotated it will cause different portions of the resistance element to be contacted by the conductive member 54.

Figs. 2 and 6 best illustrate the improvement according to the present invention. The improvement takes the fonn of an annular piece of insulating material 74 positioned on top of the plotting 62. The annular element 74 overlies the insulating member 60 of the resistance unit and its inside diameter is greater than the diameter of the circle of rotation of the pressure foot 18. As shown best in Fig. 6, the plotting compound 62 Supports the annular element 74 in vertical spaced relation with the insulating member 6% For purposes of illustration Fig. 6 exaggerates the magnitude of the spacing between the two members 60 and 74. In practice the spacing is quite small, being in the order of approximately .001 to .010 inch. Positioned between the two members 6t) and 74 is a viscous lubricant 76. Silicone greases are suitable lubricants. The viscosity of the lubricant can vary. However, it must have sufllcient viscosity to retain itself between the two members in the absence of any force tending to cause it to escape. On the other hand it must be sufficiently fluid to ooze out from between the two elements when needed. As the pressure foot 13 travels along the surface of insulating member 60, it causes that member to flex toward the resistance film. Although the flexing motion of the insulating member Gil is very slight, it is suflicient to cause escape of some of the lubricant. When a portion of insulating member 60 is flexed downward, the spacing between member 60 and annular member 74 in the region of the flexed portion is increased just enoughto permit some of the lubricant to ooze out from between the two members. Continued movement of pressure foot 18 causes different portions of insulating member 60 to be flexed and, as soon as the foot leaves a depressed portion the resiliency of the insulating member causes the depressed portion to return to its normal unflexed position. Thus in effect the lubricant is subjected to a pumping or squeezing action which not only causes some of it to escape but also causes the rest of it to be distributed evenly around the space between the two elements 60 and 74. It is believed also that capillary action is responsible in part for the even distribution of lubricant around the periphery of the top surface of the resistance unit. The lubricant that escapes is picked up by the pressure foot 18 and is distributed by the latter around the surface of insulating member 6%). The released grease lubricates the surface of insulating member 69, thereby faciliatating rotational movement of the pressure foot. It also acts to protect insulating member 60 against wear by the pressure foot.

The annular element 74 may be kept in place by various means. In the preferred embodiment shown in the drawings a relatively thin retaining ring 78 in frictional engagement with the inside surface of the housing holds the annular element 74 against the potting 62. Alternatively, the annular element may be cemented to the potting or to the housing. If desired cement may be used to hold the retaining ring 78 in place.

In practice the grease 76 is applied to the marginal portion of insulating member 6t) before the annular element 74 is applied. However, depending on its viscosity, the grease may be applied as a coating on the underside of the annular element 74 and then the latter inserted in place.

For best results the annular element 74 should be more rigid than insulating member 6i It has been found that fibreglass fabric reinforced with a suitable resin that acts as a stiffener is an excellent material from which to make element 64. However, other materials with slightly greater flexibility will also function satisfactorily.

A grease having the desired lubricating properties and viscosity can be made by mixing equal parts of Dow Corning #44 Silicone Grease (fluid grade) with Dow Corning XF-258 Silicone Fluid and Supermil ASV Grease M40. The latter is sold by Superla Products, inc, 910 South Michigan Avenue, Chicago, Illinois. Obviously other greases having the desired properties can be formulated or may be available commercially.

Various adhesives may be employed to bond the conductive element 54 to the insulating member 60. Pressure-sensitive Silicone adhesives have been found to be especially suitable for bonding metal foil to glass cloth impregnated or coated with Teflon. One suitable commercially available silicone adhesive is Dow-Corning C-269 which can be applied by knife-coating and is cured by heating.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. Therefore, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts specifically described or illustrated, and that within the scope of the appended claims it may be practised otherwise than as specifically described or illustrated.

I claim:

1. A variable resistor comprising a first sheet of rigid insulating material having on one surface thereof a deposited film resistance element, a second resilient flexible sheet of insulating material positioned parallel to said resistance element, gasket means positioned between said first and second sheets and supporting said first sheet in spaced relation with said resistance element, a metal foil contact surface attached to the inner surface of said second sheet, means sealing together said first sheet, said gasket means and the edges of said second sheet, a slidable pressure foot in engagement with the outer surface of said second sheet normally pressing a small area of said second sheet toward said resistance element to place a small area of said contact surface in engagement with said resistance element, a supply of lubricant on a limited area of the outer surface of said second sheet, and means containing said supply of lubricant and operative to release a limited quantity of said lubricant to the rest of the area of said outer surface as said pressure foot is moved along said outer surface.

2. A variable resistor comprising a first sheet of rigid insulating material having on one surface thereof a deposited film resistance element, a second resilient flexible sheet of insulating material positioned parallel to said resistance element, gasket means positioned between said first and second sheets and supporting said first sheet in spaced relation with said resistance element, a metal foil contact surface attached 'to the inner surface of said second sheet, means sealing together said first sheet, said gasket means and the edges of said second sheet, a slidable pressure foot in engagement with the outer surface of said second sheet normally pressing a small area of said second sheet toward said resistance element to place a small area of said contact surface in engagement with said resistance element, a band on the outer surface of said second sheet, said band running along the edge of said second sheet with the inner edge of said band spaced from said pressure foot, means holding said band in place, and a lubricant sandwiched between said band and said second sheet, said lubricant being sulficiently viscous to remain between said band and said second sheet so long. as the spacing between said band and second sheet remains undisturbed and sufiici'ently fluid to run out from between said band and said second sheet when said spacing is increased by downward deflection of said second sheet bysaid pressure member.

3. A variable resistor as defined by claim 2, wherein said lubricant is a silicone grease.

4. A variable resistor as defined by claim 2, wherein said second sheet consists of cloth woven from glass fibres and impregnated with a tetrafluorethylene resin.

5. A variable resistor as defined by claim 2, wherein said first sheet is constructed of quartz and said second sheet is made of cloth woven from glass fibres and impregnated with Teflon.

No references cited. 

